Abstract: In a frequency response analyzer capable of testing a system under test at a higher frequency F derived from a signal generator providing a relatively higher frequency F/K. A reference signal of frequency F-C is supplied to a mixer where it is multiplied with the return signal of frequency F to provide a fixed frequency output C. An analyzing portion of the frequency response analyzer is therefore required to operate only at this fixed frequency C and may provide an output in any suitable form, for example displays of magnitude and phase against frequency as F/K is varied. The present invention overcomes the problem of providing a VCO based signal generator capable of operation at very low frequencies.